Several attempts have been made in the past to develop a suitable membrane for the selective separation of acid gas from other gases present in a gas stream. U.S. Pat. No. 3,780,496 discloses the use of sulfonated poly(xylylene oxide) and metal salts thereof for separating CO.sub.2 from gas mixtures. The sulfonated poly(xylylene oxide) material is an ionomer with the corresponding membrane exhibiting CO.sub.2 to CH.sub.4 selectivities in the range from 28 to 50; however, CO.sub.2 to H.sub.2 selectivities are reported to be less than one.
U.S. Pat. No. 4,318,714 describes the use of ion exchange membranes for the separation of gases. It is stated that selective permeation of acid gases occurs by reversible reaction of the gas to be permeated with the counterions of the membrane. The membranes are described as being made from cross-linked, water insoluble (but water swellable) ion exchange polymer materials. Both anionic and cationic ion exchange membranes are disclosed. The data described in this patent was also disclosed in an article by LeBlanc, et al. J. Memb Sci. 6, 339-343 (1980).
J. D. Way, et al. AIChE Journal, 33, 480-487 (1987) describe the permselective properties of Nafion membranes containing monoprotonated ethylenediamine (EDAH+) counterions in addition to membranes containing Na+ ions. The EDAH+ membrane, unlike the Na+ membrane, exhibits properties consistent with permeation of CO.sub.2 by a facilitated transport mechanism and CO.sub.2 to CH.sub.4 selectivities of 88 to 550. Additionally, Way, J. D. and Noble, R. D., J. Memb. Sci. 46, 309-324 (1989) have also demonstrated the separation of CO.sub.2 from H.sub.2 S using such membranes; the presence of CO.sub.2 suppresses the permeation of H.sub.2 S and CO.sub.2 permeates preferentially with CO.sub.2 to H.sub.2 S selectivities of 5.4 to 8.3.
U.S. Pat. No. 4,758,250 discloses the use of anion exchange polyelectrolyte membranes which selectively permeate NH.sub.3 from N.sub.2 containing mixtures. As an example is cited the performance of a membrane consisting of poly(vinylammonium thiocyanate). NH.sub.3 to N.sub.2 selectivities of greater than 1000 are reported along with relatively high NH.sub.3 permeances. Also described is the fabrication and testing of an encapsulated or "sandwich" membrane in which a layer of anion exchange polyelectrolyte membrane is encapsulated between two layers of poly(trimethylsilylpropyne) (PTMSP). Such a membrane exhibited an NH.sub.3 to H.sub.2 selectivity of 6200. There is no indication or teaching of a selective permeation of gases other than ammonia.
U.S. Pat. No. 4,701,186 discloses the use of asymmetric ammonium-containing polymers. Polymers such as polysulfone or brominated polyarylene oxide were treated with ammonia or ammonium hydroxide and then exposed to a hydrohalic acid (e.g. HBr) generating an ammonium salt in the membrane. This treatment resulted in a substantial increase in selectivities accompanied by only a moderate reduction in gas permeabilities. The highest CO.sub.2 /CH.sub.4 selectivity reported was 54 but CO.sub.2 /H.sub.2 selectivities were only 0.2.
U.S. Pat. No. 4,789,386 discloses the use of ionomeric membranes in which the ionomer contains various metal cations bound to carboxylate groups. The membranes are claimed for separations involving CO.sub.2 from CH.sub.4 and N.sub.2 from O.sub.2 but selectivities of CO.sub.2 to CH.sub.4 of 6 to 8 are cited.
Itoh et al. J. Appl. Polym. Sci. 32, 3325-3343 (1986) reported the use of ethylene ionomer membranes which permeate water and CO.sub.2. The membrane consists of an ethylene-methacrylic acid copolymer. Cations, Na.sup.+ or Zn.sup.2+, were introduced by partial neutralization of the acid residues. A typical membrane contained 0.035-0.054 mole fraction of methacrylic acid with 40 to 60% of the acid neutralized. CO.sub.2 permeabilities decreased with increasing CO.sub.2 feed pressure and ranged from about 10 to 8 Barrers. No selectivity data was reported.
U.S. Pat. No. 4,741,744 discloses the use of Nafion ionomer membranes which contain hydrated metal ions as the counterion. It is stated that the membranes have improved gas permeabilities with comparable gas selectivities over the unhydrated analogs. For example, a membrane containing Ni.sup.2+ ions in the presence of water exhibits a CO.sub.2 permeance about 18 times that of Ni.sup.2+ in the absence of water but the selectivities are comparable, 32 versus 36.
Traditionally, the active separation layer of facilitated transport membranes has consisted of a liquid which reacts reversibly with a specific gas(es). The liquid is generally supported in a thin microporous matrix as for example in U.S. Pat. No. 3,819,806.
Methods have been described for the construction of multi-layer membranes in which a film of such liquid-containing microporous matrix is supported on a gas-permeable but liquid-impermeable polymer layer. U.S. Pat. No. 3,758,603 discloses the use of silicone rubber to back immobilized liquid membranes. This approach was taken one step further in U.S. Pat. No. 3,447,286 which discloses the encapsulation of an immobilized liquid membrane in between gas-permeable but liquid-impermeable polymer layers. This patent describes a membrane consisting of a liquid immobilized in the micropores of a Dacron mat, which was enclosed between the two sheets of silicone rubber. Similar membranes are disclosed in U.S. Pat. No. 4,089,653 but generally containing more than one immobilized liquid layer. More recently, U.S. Pat. No. 4,780,114 disclosed membranes consisting of molten salt hydrates supported in microporous supports and encapsulated in poly(trimethylsilylpropyne) or silicone rubber.
U.S. Pat. No. 4,758,250 discloses a water soluble ammonia permselective polymer contained or encapsulated within a water insoluble polymer which exhibits relatively high gas permeabilities. An example is given of such a multi-layer membrane consisting of an active layer of poly(vinylammonium thiocyanate) between two sheets of poly(trimethylsilylpropyne). Such a membrane is disclosed as being useful for the separation of ammonia. No indication was given that such membranes could be used to separate acid gases.